Creating a navigation service

In a typical cross-platform mobile app architecture, one would have to implement a platform-specific navigation service for each platform the app supports. In our case, Xamarin.Forms has already done this, so we're simply going to implement a single navigation service that extends the Xamarin.Forms navigation abstraction so we can perform ViewModel to ViewModel navigation.

The first thing we need to do is define an interface for our navigation service that will define its methods. We are starting with an interface so the service can be added to ViewModels via constructor injection, which we'll dive into in Chapter 4, Platform Specific Services and Dependency Injection. We are also starting with an interface so we can easily provide alternative implementations of the service without changing ViewModels that depend on it. A common scenario for this is creating a mock of the service that gets used when unit testing ViewModels.

In order to create the navigation...